The present invention relates generally to power supply controllers. More particularly, the present invention relates to synchronous rectifiers (SR) used in a power converter to simplify circuit design and improve power efficiency. A specific embodiment of the present invention relates to efficient and low-cost driving and control of a power metal-oxide-semiconductor field effect transistor (MOSFET) in a synchronous rectifier for power converters.
Switching mode power control techniques have found wide applications in computer and electronic equipment power supplies. The popularity of switching mode power supplies (SMPS) are, in part, due to their compactness, stability, efficiency, and lower cost, compared to traditional linear transformer circuits.
The flyback converter is one of the common topologies among the numerous varieties of power converters. A typical flyback converter includes a transformer having a primary winding and a secondary winding, and sometimes a third or more windings for control purposes. This transformer provides a galvanic isolation between the input and the output, and is often used in low-power low-cost power supplies.
In order to provide a DC voltage, diode rectification has been used for many years in switching mode power supplies. However, the scaling down of semiconductor technology calls for lower voltage and larger current power supply. While the diode forward drop voltage cannot be scaled down further, diode rectification can no long meet the smallness, thinness, and high-efficiency requirements desired by the consumer. Furthermore, since the circuit loss is proportional to output current, diode rectification suffers from excessive loss with large output current. For example, for low voltage drop purpose, a Schottky diode is typically employed which has a forward voltage drop of around 500 mV. For 3V output application, this limits the theoretical efficiency of a DC/DC power converter to 83 percent, even if other losses are not taken into account. For a 2V application, the efficiency is even lower and becomes unacceptable.
As a result, nowadays more and more attention is paid to synchronous rectification method using a synchronous rectifier (SR). In some examples, an SR uses a power MOSFET to replace the diode.
Even though widely used, conventional synchronous rectifiers suffer from many limitations. Theses limitations include high cost and complex circuit. More details of these and other limitations are described below.
Therefore, there is a need for methods and systems for improved synchronous rectifiers.